I realise that assignments aren't out just yet, but I've been trolling the Dear Spacer letters that have been posted and have a few ideas for pieces to exchange or have as extras if I don't get that person. My question is, as long as the piece isn't posted before the reveal date, may I use it in another challenge?

(I have just received a card from cottoncandy_bingo and the due dates there are after reveals here, so if it fits for both is it okay?)